Microorganism producing L-methionine precursor and the method of producing L-methionine precursor using the microorganism

ABSTRACT

The present invention relates to a microorganism producing L-methionine precursor, O-acetylhomoserine, and a method of producing L-methionine precursor using the microorganism.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No. 12/062,835, filed Apr. 4, 2008, which is incorporated by reference in its entirety.

STATEMENT REGARDING SEQUENCE LISTING

The Sequence Listing associated with this application is provided in text format in lieu of a paper copy, and is hereby incorporated by reference into the specification. The name of the text file containing the Sequence Listing is 200187_(—)403C1a_SEQUENCE_LISTING.txt. The text file is 23 KB, was created on Jan. 26, 2012, and is being submitted electronically via EFS-Web.

BACKGROUND

Methionine is one of essential amino acids of human body which has been widely used as feed and food additives and further used as a synthetic raw material for medical solutions and medical supplies. Methionine acts as a precursor of such compounds as choline (lecithin) and creatine and at the same time is used as a synthetic raw material for cysteine and taurine. Methionine can also provide sulfur. S-adenosyl-methionine is derived from L-methionine and plays a certain role in providing methyl group in human body and also is involved in the synthesis of various neurotransmitters in the brain. Methionine and/or S-adenosyl-L-methionine (SAM) inhibits fat accumulation in the liver and artery and alleviates depression, inflammation, liver disease, and muscle pain, etc.

The in vivo functions of methionine and/or S-adenosyl-L-methionine known so far are as follows.

1) It inhibits fat accumulation in the liver and artery promoting lipid metabolism and improves blood circulation in the brain, heart and kidney (J Hepatol. Jeon B R et al., 2001 March; 34 (3): 395-401).

2) It promotes digestion, detoxication and excretion of toxic substances and excretion of heavy metals such as Pb.

3) It can be administered as an anti-depression agent at the dosage of 800-1,600 mg/day (Am J Clin Nutr. Mischoulon D. et al., 2002 November; 76 (5): 1158S-61S).

4) It enhances liver functions (FASEB J. Mato J M., 2002 January; 16 (1): 15-26) and particularly is effective in the liver disease caused by alcohol (Cochrane Database Syst Rev., Rambaldi A., 2001; (4): CD002235).

5) It has anti-inflammatory effect on bone and joint diseases and promotes joint-recovery (ACP J Club. Sander O., 2003 January-February; 138 (1): 21, J Fam Pract., Soeken K L et al., 2002 May; 51 (5): 425-30).

6) It is an essential nutrient for hair. It provides nutrition to hair and thereby prevents hair loss (Audiol Neurootol., Lockwood D S et al., 2000 September-October; 5 (5): 263-266).

Methionine can be chemically or biologically synthesized to be applied to feed, food and medicines.

In the chemical synthesis, methionine is mostly produced by hydrolysis of 5-(β-methylmercaptoethyl)-hydantoin. The chemically synthesized methionine has a disadvantage of only being produced as a mixed form of L-type and D-type.

In the biological synthesis, methionine is produced by method the using proteins involved in methionine synthesis. L-methionine is biosynthesized from homoserine by the action of the enzyme expressed by such genes as metA, metB, metC, metE and metH, in E. coli. Particularly, metA is the gene encoding homoserine O-succinyltransferase which is the first enzyme necessary for methionine biosynthesis, and it converts homoserine into O-succinyl-L-homoserine. O-succinylhomoserine lyase or cystathionine gamma synthase encoded by metB gene converts O-succinyl-L-homoserine into cystathionine. Cystathionine beta lyase encoded by metC gene converts cystathionine into L-homocysteine. MetE encodes cobalamine-independent methionine synthase and metH encodes cobalamine-dependent methionine synthase, both of which convert L-homocysteine into L-methionine. At this time, 5,10-methylenetetrahydrofolate reductase encoded by metF and serine hydroxymethylransferase encoded by glyA work together to synthesize N(5)-methyltetrahydrofolate providing methyl group necessary for L-methionine synthesis.

L-methionine is synthesized by a series of organic reactions by the above enzymes. The genetic modification on the above proteins or other proteins affecting the above proteins might result in the increase of L-methionine synthesis. For example, Japanese Laid-Open Patent Publication No. 2000/139471 describes a method of producing L-methionine with the Escherichia sp. of which thrBC and metJ genes on the genome are deleted, metBL is over-expressed and metK is replaced by a leaky mutant. Also, US Patent Publication No. US2003/0092026 A1 describes a method using a metD (L-methionine synthesis inhibitor) knock-out microorganism which belongs to Corynerbacterium sp. US Patent Publication No. US2002/0049305 describes a method to increase L-methionine production by increasing the expression of 5,10-methylenetetrahydrofolate reductase (metF).

The methionine produced by the biological method is L-type, which has advantages but the production amount is too small. This is because the methionine biosynthetic pathway has very tight feed-back regulation systems. Once methionine is synthesized to a certain level, the final product methionine inhibits the transcription of metA gene encoding the primary protein for initiation of methionine biosynthesis. Over-expression of metA gene itself cannot increase methionine production because the metA gene is suppressed by methionine in the transcription stage and then degraded by the intracellular proteases in the translation stage (Dvora Biran, Eyal Gur, Leora Gollan and Eliora Z. Ron: Control of methionine biosynthesis in Escherichia coli by proteolysis: Molecular Microbiology (2000) 37 (6), 1436-1443). Therefore, many of previous patents were focused on how to free the metA gene from its feed-back regulation system (WO2005/108561, WO1403813).

US patent publication No. US2005/0054060A1 describes a method to synthesize homocysteine or methionine by modified cystathionine synthase (O-succinylhomoserine lyase) which use methylmercaptan (CH₃SH) or hydrogen sulfide (H2S) directly as a sulfur source, not cysteine. However, it is well understood by those in the art that cystathionine synthase can bind various methionine precursor in the cells and thereby produce by-product at high level. For example, it was reported that cystathionine synthase accumulate high levels of homolanthionin by side reaction of O-succinylhomoserine and homocystein (J. Bacteriol (2006) vol 188:p 609-618). Therefore, overexpression of cystathionine synthase can reduce the efficiency of Intracellular reaction due to the increase of their side reaction. In addition, this method has many disadvantages. This process uses intracellular metabolic pathways which have side reactions and feed back regulation systems. Also this process uses H₂S or CH₃SH which has a severe cytotoxity to cells. Hence the methionine production yield is comparatively small.

To solve these problems, the present inventor had developed two step process comprising; first step of producing of L-methionine precursor by E. coli fermentation; and second step of converting L-methionine precursor into L-methionine by enzyme reaction (PCT/KR2007/003650). This two step process can solve the above problems, such as cytotoxicity of sulfides, feed-back regulation by methionine and SAMe, decomposition of intermediate product by intracellular enzymes (e.g. cystathionine gamma synthase, O-succinylhomoserine sulfhydrylase and O-acetylhomoserine sulfhydrylase). Moreover, against the chemical methionine synthetic method which produce mixed form of D-methionine and L-methionine, the two step process is very efficient to produce only L-methionine selectively.

In this two step process, production yield of methionine precursor is one of the key factor for the increase of methionine production yield. To increase the synthetic yield of methionine precursor, O-acetyl homoserine, good combination of strong aspartokinase, homoserine transferase and O-acetyl homoserine transferase is really important. On the above-mentioned background, the present inventors were constructed the L-methionine precursor producing strain which is characterized by the followings; a) the homoserine O-acetyltransferase activity (EC2.3.1.31) is introduced and enhanced by the integration of genes selected from corynebacterium sp., Leptospira sp., Deinococcus sp., Pseudomonas sp., or Mycobacterium sp.;

or b) the aspartokinase or homoserine dehydrogenase activity (EC2.7.2.4 or 1.1.1.3) is enhanced,

or c) combination of a) and b).

SUMMARY

The present invention provides a microorganism producing L-methionine precursor and a method of producing L-methionine precursor using the microorganism.

More particularly, the present invention provides a L-methionine precursor producing strain which is characterized by the followings; a) the homoserine O-acetyltransferase activity (EC2.3.1.31) is introduced and enhanced by the integration of genes selected from corynebacterium sp., Leptospira sp., Deinococcus sp., Pseudomonas sp., or Mycobacterium sp.;

or b) the aspartokinase or homoserine dehydrogenase activity (EC2.7.2.4 or 1.1.1.3) is enhanced,

or c) combination of a) and b),

and a method of producing L-methionine precursor using the strain.

BRIEF DESCRIPTION OF THE DRAWINGS

The application of the preferred embodiments of the present invention is best understood with reference to the accompanying drawings, wherein:

FIG. 1 is a diagram illustrating genetic manipulation of the methionine precursor producing strain.

FIG. 2 is a diagram illustrating chemical structures of 2 step process for the production of methionine.

FIG. 3 is a schematic diagram of pCJ-MetX-CL for the expression of metX gene.

DETAILED DESCRIPTION

In accordance with an aspect thereof, the present invention is directed to a L-methionine precursor-producing strain characterized by the followings; a) the homoserine O-acetyltransferase activity (EC2.3.1.31) is introduced and enhanced by the integration of genes selected from corynebacterium sp., Leptospira sp., Deinococcus sp., Pseudomonas sp., or Mycobacterium sp.;

or b) the aspartokinase or homoserine dehydrogenase activity (EC2.7.2.4 or 1.1.1.3) is enhanced,

or c) combination of a) and b),

and a method of producing L-methionine precursor using the strain.

The term “L-methionine precursor” is defined as metabolites that are part of the methionine specific metabolic pathway or can be derived of these metabolites. Particularly, L-methionine precursor as used herein refers to a O-acetylhomoserine.

The term “L-methionine precursor-producing strain”, as used herein refers to a prokaryotic or eukaryotic microorganism strain that is able to accumulate L-methionine precursor by manipulation according to the present invention. For example, the strain can be selected from the group consisting of Escherichia sp., Erwinia sp., Serratia sp., Providencia sp., Corynebacteria sp., Pseudomonas sp., Leptospira sp., Salmonellar sp., Brevibacteria sp., Hypomononas sp., Chromobacterium sp. and Norcardia sp. microorganisms or fungi or yeasts. Preferably, the microorganisms of Escherichia sp., Corynebacterium sp., Leptospira sp. and yeasts can be used to produce O-acetylhomoserine. More preferably, the microorganisms of Escherichia sp. can be used, and most preferably Escherichia coli (hereinafter referred to as “E. coli”) can be used.

In various embodiments, the present invention provides an L-methionine precursor-producing strain in which a gene involved in the decomposition of authentic O-succinyl homoserine or O-acetylhomoserine is deleted or weakened and instead a gene involved in the synthesis of O-acetylhomoserine is introduced or enhanced. The present invention also selectively provides a strain in which threonine biosynthesis pathway is blocked or weakened to enhance O-acetylhomoserine production. The present invention further provides a strain in which homoserine O-acetyltransferase free from feed back regulation system is introduced, over-expressed or activiy-enhanced. The present invention further provides a strain in which aspartokinase or homoserine dehydrogenase is over-expressed or activiy-enhanced. The present invention also provides a strain in which homoserine O-acetyltransferase free from feed back regulation system is introduced, over-expressed or activiy-enhanced and aspartokinase or homoserine dehydrogenase is over-expressed or activiy-enhanced.

More particularly, the present invention provides an L-methionine precursor producing strain by deleting metB gene involved in the decomposition of L-methionine precursor, thrB gene involved in threonine biosynthesis pathway and metJ gene regulating the transcription of L-methionine precursor production genes. The present invention also provides an L-methionine precursor producing strain by knocking-out authentic metA or metX gene involved in the synthesis of methionine precursor and by introducing foreign metX. The present invention also provides an L-methionine precursor producing strain by enhancing the activity encoded by the thrA gene.

More preferably, the present invention also provides an L-methionine precursor producing strain by knocking-out authentic metA or metX gene involved in the synthesis of methionine precursor and by introducing foreign metX gene free from feed-back system and by enhancing the activity encoded by the thrA gene.

In the present invention, “inactivation” as used herein refers to a deletion or an attenuation of the gene. A deletion of the gene is performed by cutting out of a region of the gene or modifying the protein sequence by introducing a specific gene sequence on the chromosome. The term “attenuation” or “weakening” in this connection describes the reduction or elimination of the intracellular activity of one or more enzymes (proteins) in a microorganism which are encoded by the corresponding DNA, for example by reducing the activity of the protein by modifying a promoter region of the gene and the nucleotide sequence of 5′-UTR or by introducing the mutation in the ORF region of the target gene. By attenuation measures, the activity or concentration of the corresponding protein is in general reduced to 0 to 75%, 0 to 50%, 0 to 25%, 0 to 10% or 0 to 5% of the activity or concentration of the wild-type protein or of the activity or concentration of the protein in the starting microorganism.

To achieve an attenuation, for example, expression of the gene or the catalytic properties of the enzyme proteins can be reduced or eliminated. The two measures can optionally be combined.

The reduction in gene expression can take place by suitable culturing, by genetic modification (mutation) of the signal structures of gene expression or also by the antisense-RNA technique. Signal structures of gene expression are, for example, repressor genes, activator genes, operators, promoters, attenuators, ribosome binding sites, the start codon and terminators. The expert can find information in this respect, inter alia, for example, in Jensen and Hammer (Biotechnology and Bioengineering 58: 191 195 (1998)), in Carrier and Keasling (Biotechnology Progress 15: 58 64 (1999)), Franch and Gerdes (Current Opinion in Microbiology 3: 159 164 (2000)) and in known textbooks of genetics and molecular biology, such as, for example, the textbook of Knippers (“Molecular Genetics”, 6th edition, 1995) or that of Winnacker (“Genes and Clones”, 1990).

Mutations which lead to a change or reduction in the catalytic properties of enzyme proteins are known from the prior art. Examples which may be mentioned are the works of Qiu and Goodman (Journal of Biological Chemistry 272: 8611 8617 (1997)), Yano et al. (Proceedings of the National Academy of Sciences, USA 95: 5511 5515 (1998)), and Wente and Schachmann (Journal of Biological Chemistry 266: 20833 20839 (1991)). Summarizing descriptions can be found in known textbooks of genetics and molecular biology, such as e.g. that by Hagemann (“General Genetics”, 1986).

Possible mutations are transitions, transversions, insertions and deletions. Depending on the effect of the amino acid exchange on the enzyme activity, “missense mutations” or nonsense mutations are referred to. Insertions or deletions of at least one base pair in a gene lead to “frame shift mutations”, which lead to incorrect amino acids being incorporated or translation being interrupted prematurely. If a stop codon is formed in the coding region as a consequence of the mutation, this also leads to a premature termination of the translation. Deletions of several codons typically lead to a complete loss of the enzyme activity. Instructions on generation of such mutations are prior art and can be found in known textbooks of genetics and molecular biology, such as e.g. the textbook by Knippers (“Molecular Genetics”, 6th edition, 1995), that by Winnacker (“Genes and Clones”, 1990) or that by Hagemann (“General Genetics”, 1986).

Suitable mutations in the genes, such as, for example, deletion mutations, can be incorporated into suitable strains by gene or allele replacement.

In the present invention, the term “enhancement” describes the increase in the intracellular activity of an enzyme which is encoded by the corresponding DNA. The enhancement of intracellular activity of an enzyme can be achieved by the overexpression of the gene. Overexpression of the target gene can be achieved by modifying the promoter region of the gene or the nucleotide sequence of the 5′-UTR region. Overexpression of the target gene can also be achieved by introducing the extra copy of the target gene on the chromosome, by transforming the host strain with the vector containing the target gene with a promoter, or by introducing the mutation which can increase the expression in the target gene. The enhancement of the intracellular activity of an enzyme can also be achieved by introducing the mutation in the ORF region of the target gene. By enhancement measures, the activity or concentration of the corresponding protein is in general increased by at least 10%, 25%, 50%, 75%, 100%, 150%, 200%, 300%, 400% or 500%, up to a maximum of 1000% or 2000%, based on that of the wild-type protein or the activity or concentration of the protein in the starting microorganism.

In a preferred embodiment of the present invention, the method for preparing an L-methionine precursor producing strain is as follows;

In step 1, a gene encoding such proteins as cystathionine gamma synthase, O-succinylhomoserine sulfhydrylase or O-acetylhomoserine sulfhydrylase is deleted or weakened in a strain in order to accumulate L-methionine precursor such as O-succinyl homoserine or O-acetyl homoserine.

The gene encoding cystathionine gamma synthase is indicated as metB, the gene encoding O-succinylhomoserine sulfhydrylase is indicated as metZ, and the gene encoding O-acetylhomoserine sulfhydrylase is indicated as metY. A gene encoding the protein having the above mentioned activity is exemplified by metB in Escherichia coli. The genomic sequence of the gene can be obtained from the genomic sequence of E. coli (Accession no. AAC75876) informed in the previous report (Blattner et. al., Science 277: 1453-1462 (1997)). The above genomic sequence also can be obtained from NCBI (National Center for Biotechnology Information) and DDBJ (DNA Data Bank Japan). Other genes having the same activity are exemplified by metB and metY derived from Corynebacterium, and metZ derived from Pseudomonas.

Cystathionine gamma synthase or O-succinylhomoserine sulfhydrylase or O-acetylhomoserine sulfhydrylase has the activity to convert O-succinyl homoserine or O-acetylhomoserine into cystathionine or homocysteine as shown in the following reaction formulas. Therefore, where a gene having this activity is deleted or weakened, O-succinylhomoserine or O-acetylhomoserine is excessively accumulated in the culture solution. L-cysteine+O-succinyl-L-homoserine<=>succinate+cystathionine L-cysteine+O-acetyl-L-homoserine<=>acetate+cystathionine HS⁻+O-succinyl-L-homoserine<=>succinate+homocysteine HS⁻+O-acetyl-L-homoserine<=>acetate+homocysteine

In step 2, thrB gene of encoding homoserine kinase in the strain prepared in step 1 is deleted or weakened. The thrB gene is involved in the synthesis of O-phosphohomoserine from homoserine, which is then converted into threonine by thrC gene. The thrB gene is deleted or weakened in order to use all the produced homoserine for the synthesis of methionine precursor.

In step 3, a transcription regulator of methionine synthetic pathway is deleted or weakened. The metA, metB, metC, metE, and metF gene involved in the methionine synthesis is repressed by feed-back regulation system. The metJ gene is a typical transcriptor regulator gene in E. coli. To let the metA or metX gene be over-expressed to synthesize methionine precursor, metJ needs to be eliminated. Therefore, metJ gene is eliminated in E. coli, the metA or metX gene expression is always increased, leading to the mass-production of L-methionine precursor.

The above steps 2 and 3 can be modified or eliminated according to a precursor producing strain. However, it can be more preferably executed to enhance the precursor production pathway in the microorganism of Escherichia sp.

In step 4, metX gene encoding homoserine O-acetyltransferase which mediate the first process of methionine biosynthesis pathway is introduced in order to increase synthesis of O-acetylhomoserine, L-methionine precursor. The metX is a common designation of gene encoding the protein having activity of homoserine O-acetyltransferase, and a novel foreign homoserine O-acetyltransferase can be obtained from various microorganism species. For example, the homoserine O-acetyltransferase peptide is encoded by the genes selected from the corynebacterium sp., Leptospira sp., Deinococcus sp., Pseudomonas sp., or Mycobacterium sp. Preferably, homoserine O-acetyltransferase peptide is encoded by the genes selected from corynebacterium glutamicum, Leptospira meyeri, Deinococcus radiodurans, Pseudomonas aeruginosa, or Mycobacterium smegmatis. More preferably, the homoserine O-acetyltransferase peptide is encoded by the genes selected from Unipro database No. Q9RVZ8 (Deinococcus radiodurans), NP_(—)249081 (Pseudomonas aeruginosa), or YP_(—)886028 (Mycobacterium smegmatis). The metX gene from leptospira meyeri was known as feed back resistant (J Bacteriol. 1998 January; 180 (2):250-5. Belfaiza J et al.) and several homoserine O-acetyltransferase were confirmed as feed back resistant in our lab before.

The introduction and enhancement of the metX gene can be performed by the introduction of the gene or by the modification of a promoter region of the gene and the nucleotide sequence of 5′-UTR or by introducing the mutation in the ORF region of the target gene. The enhancement of metX gene expression results in the increase of methionine precursor synthesis.

In step 5, aspartokinase or homoserine dehydrogenase is activity-enhanced in order to increase synthesis of homoserine which is the precursor of O-succinyl homoserine or O-acetyl homoserine. The thrA is a common designation of gene encoding the peptide having activity of aspartokinase and homoserine dehydrogenase. Preferably, an aspartokinase and homoserine dehydrogenase encoded by the gene from Unipro database No: AP_(—)000666. More preferably, the amino acid sequence encoded by the thrA gene is represented in SEQ. ID. NO: 27 that has mutation in amino acid position 345. Enhancement of the thrA activity is performed by introducing the mutation in the thrA gene or by further introducing the target gene on the chromosome or by further introducing processed plasmid.

O-acetylhomoserine which is L-methionine precursor, can be accumulated in a strain by taking advantage of a part or the entire process of the above step 1 to step 5.

The L-methionine precursor-producing strain can be prepared from the strain producing L-lysine, L-threonine or L-isoleucine. Preferably, it can be prepared by using the L-threonine producing strain. With this strain, homoserine synthesis is going easier and the production of methionine precursor is resultantly increased. So, methionine precursor can be accumulated by deleting or weakening a gene involved in threonine biosynthesis pathway and then metA or metY or MetZ gene, using the L-threonine producing strain. It is more preferred to delete or weaken thrB gene first and then metB, metY or metZ to synthesize methionine precursor. In the meantime, the enhancement of metX gene expression results in the increase of methionine precursor synthesis.

The term, “L-threonine-producing strain” of the invention refers to a prokaryotic or eukaryotic microorganism strain that is able to produce L-threonine in vivo. For example, the strain can be include L-threonine producing microorganism strains belongs to Escherichia sp., Erwinia sp., Serratia sp., Providencia sp., Corynebacterium sp. and Brevibacterium sp. Among these, Escherichia sp. microorganism is preferred and Escherichia coli is more preferred.

In a preferred embodiment of the present invention, CJM002, the L-threonine producing and L-methionine-independent strain transformed from TF4076 (KFCC 10718, Korean Patent No. 92-8365), the L-threonine producing E. coli mutant strain, was used. TF4076 has a requirement for methionine, and is resistant to methionine analogues (ex, α-amino-β-hydroxy valeric acid, AHV), lysine analogues (ex, S-(2-aminoethyl)-L-cysteine, AEC), and isoleucine analogues (ex, α-aminobutylic acid). The TF4076 is not able to synthesize methionine in vivo because it is the strain which has a requirement for methionine. To use this strain as the methionine precursor producing strain of the invention by free from a requirement for methionine, the present inventors prepared the L-threonine producing strain E. coli CJM002 free from the requirement for methionine by artificial mutation using NTG. The E. coli CJM002 was named as Escherichia coli MF001 and deposited at KCCM (Korean Culture Center of Microorganism, Eulim Buld., Hongje-1-Dong, Seodaemun-Ku, Seoul, 361-221, Korea) on Apr. 9, 2004 (Accession No: KCCM-10568). In the present invention, the metB, thrB, metJ and metA gene of the E. coli CJM002 were deleted, then the metX gene was introduced in the metA locus. The resulting L-methionine precursor producing strain constructed using E. coli CJM002 was named CJM-X. The metX gene of CJM-X strain derived from D. radiodurans. The CJM-X strain was transformed with the thrA expression vector, and was named CJM-X (pthrA(M)-CL).

The Escherichia coli CJM-X (pthrA(M)-CL), O-acetylhomoserine producing strain, prepared by the above method was deposited at KCCM (Korean Culture Center of Microorganism, Eulim Buld., Hongje-1-Dong, Seodaemun-Ku, Seoul, 361-221, Korea) on Jan. 23, 2008, with Accession No. KCCM 10921P.

In another preferred embodiment of the present invention, FTR2533 which is the L-threonine producing strain disclosed in PCT/KR2005/00344 was used. FTR2533 was derived from Escherichia coli TFR7624 which was derived from Escherichia coli Accession No. KCCM10236. And Escherichia coli Accession No. KCCM 10236 which was derived from Escherichia coli TF4076. Escherichia coli Accession No. KCCM 10236 is, capable of expressing higher levels of the ppc genes catalyzing the formation oxaloacetate from PEP and the enzymes necessary for threonine biosynthesis from aspartate e.g. thrA: aspartokinaze 1-homoserine dehydrogenase, thrB: homoserine kinase, thrC: threonine synthase, thereby enabling an increase in L-threonine production. And Escherichia coli FTR7624(KCCM10538) have an inactivated tyrR gene which regresses the expression of tyrB gene necessary for L-threonine biosynthesis. And Escherichia coli FTR2533 (KCCM10541) is the L-threonine producing strain having an in-activated gaIR gene, the L-threonine producing E. coli mutant strain.

In the present invention, the metB, thrB, metJ and metA gene of the E. coli FTR2533 were deleted, then the metX gene was introduced in the metA locus. The result L-methionine precursor producing strain constructed using E. coli FTR2533 was named CJM2-X. The metX gene of CJM2-X strain derived from D. radiodurans. The CJM2-X strain was transformed with the thrA expression vector, and was named CJM2-X (pthrA(M)-CL).

The Escherichia coli CJM2-X (pthrA(M)-CL) was deposited at KCCM (Korean Culture Center of Microorganism, Eulim Buld., Hongje-1-Dong, Seodaemun-Ku, Seoul, 361-221, Korea) on Feb. 12, 2008, with Accession No. KCCM 10925P.

In another aspect, the present invention relates to a method of producing L-methionine precursor, the method comprising: a) fermentation of the above microorganisms; b) enrichment of the L-methionine precursor in the medium or in the microorganisms; and c) isolation of the L-methionine precursor.

The culture of the L-methionine precursor producing strain prepared above can be performed by a proper medium and conditions known to those in the art. It is well understood by those in the art that the culture method can be used by easily adjusting, according to the selected strain. For example, the culture method includes, but not limited to batch, continuous culture and fed-batch. A variety of culture methods are described in the following reference: “Biochemical Engineering” by James M. Lee, Prentice-Hall International Editions, pp 138-176.

The medium has to meet the culture conditions for a specific strain. A variety of microorganism culture mediums are described in the following reference: “Manual of Methods for General Bacteriology” by the American Society for Bacteriology, Washington D.C., USA, 1981. Those mediums include various carbon sources, nitrogen sources and trace elements. The carbon source is exemplified by carbohydrate such as glucose, sucrose, lactose, fructose, maltose, starch, cellulose; fat such as soybean oil, sunflower oil, castor oil and coconut oil; fatty acid such as palmitic acid, stearic acid, and linoleic acid; alcohol such as glycerol and ethanol; and organic acid such as acetic aid. One of these compounds or a mixture thereof can be used as a carbon source. The nitrogen source is exemplified by such organic nitrogen source as peptone, yeast extract, gravy, malt extract, corn steep liquor (CSL) and bean flour and such inorganic nitrogen source as urea, ammonium sulfate, ammonium chloride, ammonium phosphate, ammonium carbonate and ammonium nitrate. One of these compounds or a mixture thereof can be used as a nitrogen source. The medium herein can additionally include potassium dihydrogen phosphate, dipotassium hydrogen phosphate and corresponding sodium-containing salts as a phosphate source. The medium also can include a metal salt such as magnesium sulfate or iron sulfate. In addition, amino acids, vitamins and proper precursors can be added as well. The mediums or the precursors can be added to the culture by batch-type or continuous type.

PH of the culture can be adjusted during the cultivation by adding in the proper way such a compound as ammonium hydroxide, potassium hydroxide, ammonia, phosphoric acid and sulfuric acid. The generation of air bubbles can be inhibited during the cultivation by using an antifoaming agent such as fatty acid polyglycol ester. To maintain aerobic condition of the culture, oxygen or oxygen-containing gas (ex, air) can be injected into the culture. The temperature of the culture is conventionally 20-45° C., preferably 25-40° C. The period of cultivation can be continued until the production of L-methionine precursor reaches a wanted level, and the preferable cultivation time is 10-160 hours.

EXEMPLIFICATION

Practical and presently preferred embodiments of the present invention are illustrative as shown in the following Examples which are set forth to illustrate, but are not to be construed as the limit of the present invention. It will be appreciated that those skilled in the art, on consideration of this disclosure, may make modifications and improvements within the spirit and scope of the present invention.

Example 1 Construction of a Methionine Precursor Producing Strain

<1-1> Deletion of metB Gene

To delete the metB gene encoding cystathionine synthase in E. coli strain, FRT-one-step PCR deletion was performed (PNAS (2000) vol 97: P6640-6645). Primers of SEQ. ID. NO: 1 and NO: 2 were used for PCR using pKD3 vector (PNAS (2000) vol 97: P6640-6645) as a template, resulting in the construction of deletion cassette. PCR was performed as follows; 30 cycles of denaturation at 94° C. for 30 seconds, annealing at 55° C. for 30 seconds, extension at 72° C. for 1 minute.

The PCR product was electrophoresised on 1.0% agarose gel, followed by purification of DNA obtained from 1.2 kbp band. The recovered DNA fragment was electroporated into E. coli (K12) W3110 transformed with pKD46 vector (PNAS (2000) vol 97: P 6640-6645). Before electroporation, W3110 transformed with pKD46 was cultivated at 30° C. in LB medium containing 100 μg/L of ampicillin and 5 mM of L-arabinose until OD₆₀₀ reached 0.6. Then, the cultured strain was washed twice with sterilized distilled water and one more time with 10% glycerol. Electroporation was performed at 2500 V. The recovered strain was streaked on LB plate medium containing 25 μg/L of chloramphenichol, followed by culture at 37° C. for overnight. Then, a strain exhibiting resistance to chloramphenichol was selected.

PCR was performed by using the selected strain as a template with the primers No1 and 2 under the same condition. The deletion of metB gene was identified by confirming the 1.2 kb sized gene on 1.0% agarose gel. The strain was then transformed with pCP20 vector (PNAS (2000) vol 97: P6640-6645) and cultured in LB medium to eliminate the chloramphenichol marker gene. The final metB knock-out strain was constructed in which the size of metB gene reduced to 150 bp on 1.0% agarose gel by PCR under the same conditions. The constructed strain was named W3-B.

<1-2> Deletion of thrB Gene

The inventors tried to increase O-acetylhomoserine synthesis from homoserine by deletion of thrB gene encoding homoserine kinase. Particularly, when the threonine producing strain is used as a production host of O-acetylhomoserine, deletion of thrB gene is necessary because conversion of homoserine to O-phoshohomoserine by this gene is very strong. To delete thrB gene in the W3-B strain constructed above, FRT one step PCR deletion was performed by the same manner as described above for the deletion of metB gene.

To construct thrB deletion cassette, PCR was performed by using pKD4 vector (PNAS (2000) vol97: P6640-6645) as a template with primers of SEQ. ID. NO: 3 and NO: 4 as follows; 30 cycles of denaturation at 94° C. for 30 seconds, annealing at 55° C. for 30 seconds, extension at 72° C. for 1 minute. The PCR product was electrophoresed on 1.0% agarose gel, followed by purification of DNA obtained from 1.6 kbp band. The recovered DNA fragment was electroporated into the W3-B strain transformed with pKD46 vector. The recovered strain was streaked on LB plate medium containing 50 μg/L of kanamycin, followed by culture at 37° C. for overnight. Then, a strain exhibiting resistance was selected.

PCR was performed by using the selected strain as a template with primers of SEQ. ID. NO: 3 and NO: 4 under the same conditions as the above. The deletion of ThrB gene was identified by selecting the strain whose size is 1.6 kb on 1.0% agarose gel. The strain was then transformed with pCP20 vector and cultured in LB medium. The final thrB knock out strain was constructed in which the size of thrB gene reduced to 150 kb on 1.0% agarose gel by PCR under the same conditions. Kanamycin marker was confirmed to be eliminated. The constructed strain was named W3-BT.

<1-3> Deletion of metJ Gene

To delete the metJ gene which is the regulator gene of the metA gene involved in methionine precursor synthesis, FRT one step PCR deletion was performed by the same manner as used for the deletion of metB gene.

To construct a metJ deletion cassette, PCR was performed with primers of SEQ. ID. NO: 5 and NO: 6 as follows; 30 cycles of denaturation at 94° C. for 30 seconds, annealing at 55° C. for 30 seconds, extension at 72° C. for 1 minute.

The PCR product was electrophoresed on 1.0% agarose gel, followed by purification of DNA obtained from 1.2 kbp band. The recovered DNA fragment was electroporated into the W3-BT strain transformed with pKD46 vector. The recovered strain was streaked on LB plate medium containing chloramphenicol, followed by culture at 37° C. overnight. Then, a strain exhibiting resistance was selected.

PCR was performed by using the selected strain as a template with primers of SEQ. ID. NO: 7 and NO: 8 under the same conditions as the above. The deletion of metJ was identified by confirming the 1.6 kb sized gene on the 1.0% agarose gel. The strain was then transformed with pCP20 vector and cultured in LB medium. The final metJ knock out strain was constructed in which the size of metJ gene reduced to 600 kb on 1.0% agarose gel by PCR under the same conditions and the strain Chloramphenicol marker was confirmed to be eliminated. The constructed strain was named W3-BTJ.

<1-4> Deletion of metA Gene

To increase the production of O-acetylhomoserine, metA gene encoding homoserine O-succinyltransferase was deleted in W3-BTJ strain. Based on the founding that metX gene integration resulted in the accumulation of both O-succinylhomoserine and O-Acetylhomoserine, it was expected that metA gene deletion resulted in the promotion of the accumulation of O-acetylhomoserine (Table 3). To delete the metA gene, FRT one step PCR deletion was performed.

To construct metA deletion cassette, PCR was performed with primers of SEQ. ID. NO: 9 and NO: 10 as follows; 30 cycles of denaturation at 94° C. for 30 seconds, annealing at 55° C. for 30 seconds, extension at 72° C. for 1 minute.

The PCR product was electrophoresed on 1.0% agarose gel, followed by purification of DNA obtained from 1.2 kbp band. The recovered DNA fragment was electroporated into the E. coli W3-BTJ strain transformed with pKD46 vector. The recovered strain was streaked on LB plate medium containing chloramphenicol, followed by culture at 37° C. for overnight. Then, a strain exhibiting resistance was selected.

PCR was performed by using the selected strain as a template with primers of SEQ. ID. NO: 9 and NO: 10 under the same conditions as the above. The deletion of metA gene was identified by confirming 1.1 kb sized gene on 1.0% agarose gel. The strain was then transformed with pCP20 vector and cultured in LB medium. The final metA knock out strain was constructed in which the size of metA gene reduced to 100 kb on 1.0% agarose gel by PCR under the same conditions. Chloramphenicol marker was confirmed to be eliminated. The constructed strain was named W3-BTJA.

<1-5> Overexpression of metX Gene

To increase production of O-acetylhomoserine, overexpression of metX gene encoding homoserine O-acetyltransferase was performed.

PCR was performed by using chromosome of Leptospira meyeri as a template with primers of SEQ. ID. NO: 28 and NO: 29 as follows; 25 cycles of denaturation at 94° C. for 30 seconds, annealing at 55° C. for 30 seconds, extension at 72° C. for 2 minute.

The PCR product was electrophoresed on 1.0% agarose gel, followed by purification of DNA obtained from 1.1 kbp band. After isolation of the DNA fragment, vector pCL1920 containing CJ1 promoter was cleaved with the restriction enzyme EcoRV and ligated to the isolated DNA fragment. The E. coli was transformed with the vector and plasmid-carrying cells were selected on LB agar containing 50 μg/L of spectinomycin. The constructed vector was named pCJ1-MetXlme-CL. The vector was electroporated into W3-BTJ strain and the constructed strain was named W3-BTJ/pCJ-MetXlme-CL.

As another method of overexpressing of metX gene, PCR was performed by using chromosome of Corynebacterium glutamicum as a template with primers of SEQ. ID. NO: 30 and NO: 31 as follows; 25 cycles of denaturation at 94° C. for 30 seconds, annealing at 55° C. for 30 seconds, extension at 72° C. for 2 minute.

The PCR product was electrophoresed on 1.0% agarose gel, followed by purification of DNA obtained from 1.1 kbp band. After isolation of the DNA fragment, vector pCL1920 containing CJ1 promoter was cleaved with the restriction enzyme EcoRV and ligated to the isolated DNA fragment. The E. coli was transformed with the vector and plasmid-carrying cells were selected on LB agar containing 50 μg/L of spectinomycin. The constructed vector was named pCJ1-MetXlme-CL. The vector was electroporated into W3-BTJ strain and the constructed strain was named W3-BTJ/pCJ-MetXcgl-CL.

As another method of overexpressing of metX gene, PCR was performed by using chromosome of Deinococcus radiodurans as a template with primers of SEQ. ID. NO: 11 and NO: 12 as follows; 25 cycles of denaturation at 94° C. for 30 seconds, annealing at 55° C. for 30 seconds, extension at 72° C. for 2 minute.

The PCR product was electrophoresed on 1.0% agarose gel, followed by purification of DNA obtained from 1.1 kbp band. After isolation of the DNA fragment, vector pCL1920 containing CJ1 promoter was cleaved with the restriction enzyme EcoRV and ligated to the isolated DNA fragment. The E. coli was transformed with the vector and plasmid-carrying cells were selected on LB agar containing 50 μg/L of spectinomycin. The constructed vector was named pCJ1-MetXdr-CL. The vector was electroporated into W3-BTJ strain and the constructed strain was named W3-BTJ/pCJ-MetXdr-CL.

As another method of overexpressing of metX gene, PCR was performed by using chromosome of Mycobacterium smegmatis as a template with primers of SEQ. ID. NO: 13 and NO: 14 as follows; 25 cycles of denaturation at 94° C. for 30 seconds, annealing at 55° C. for 30 seconds, extension at 72° C. for 2 minute.

The PCR product was electrophoresed on 1.0% agarose gel, followed by purification of DNA. After isolation of the DNA fragment, vector pCL1920 containing CJ1 promoter was cleaved with the restriction enzyme EcoRV and ligated to the isolated DNA fragment. The E. coli was transformed with the vector and plasmid-carrying cells were selected on LB agar containing 50 μg/L of spectinomycin. The constructed vector was named pCJ-MetXmsm-CL. The vector was electroporated into W3-BTJ strain and the constructed strain was named W3-BTJ/pCJ-MetXmsm-CL.

As another method of overexpressing of metX gene, PCR was performed by using chromosome of Pseudomonas aeruginosa PAO1(pae) as a template with primers of SEQ. ID. NO: 15 and NO: 16 as follows; 25 cycles of denaturation at 94° C. for 30 seconds, annealing at 55° C. for 30 seconds, extension at 72° C. for 2 minute.

The PCR product was electrophoresed on 1.0% agarose gel, followed by purification of DNA. After isolation of the DNA fragment, vector pCL1920 containing CJ1 promoter was cleaved with the restriction enzyme EcoRV and ligated to the isolated DNA fragment. The E. coli was transformed with the vector and plasmid-carrying cells were selected on LB agar containing 50 μg/L of spectinomycin. The constructed vector was named pCJ-MetXpae-CL.

The above constructed vectors were electroporated into W3-BTJA strain respectively and production capacity of the strain was checked using flask culture as described in Example 2-1.

O-acetylhomoserine production of each strain is measured, based on 100% of O-acetylhomoserine production of strain introduced metX vector derived from corynebacterium glutamicum (PCT/KR2007/003650).

As a result, O-acetylhomoserine production capacity was significantly increased in the strains each transformed with vector pCJ-MetXdra-CL, pCJ-MetXmsm-CL and pCJ-MetXlme-CL.

O-acetylhomoserine Plasmid production (%) W3BTJA pCJ-metXcgl-CL 100 pCJ-metXmsm-CL 114 pCJ-metXlme-CL 105 pCJ-metXpae-CL 100 pCJ-metXdra-CL 164

-   -   For the stable expression of the metX gene from D. radiodurans         (metX(dra)) showing the most efficient production among the         above three, the metX(dra) gene was inserted into the chromosome         of E. coli. pSG vector system was used to construct         metX(dra)-inserting strain (Appl. Environ. Microbiol. 1993 V59.         p.3485-3487, Villaverde. A et. al).

First of all, 600 bp of upstream region of the metA from genomic DNA was PCR-amplified with primers of SEQ. ID. NO: 17 and NO: 18, and 600 bp of downstream region of the metA from genomic DNA was PCR-amplified with primers of SEQ. ID. NO: 19 and NO: 20. Then, the metX(dra) was PCR-amplified with primers of SEQ. ID. NO: 21 and NO: 22. Each PCR fragment was isolated by gel-elution and the fragment mixture was PCR-amplified with primers of SEQ. ID. NO: 23 and NO: 24. The amplified DNA cleaved with the restriction enzyme BamHI/EcoRI and cloned into pSG76C vector. W3-BTJA strain was transformed with the above constructed vector and chloramphenichol-resistant strains were selected and successful cloning of metX dra gene was detected. The detected strains were transformed with plscel vector to removing marker, and reselected. As a result of confirming O-acetylhomoserine production capacity, the production of the metX(dra) gene-introduced strain was similar to that strain harboring pCL-metXdra plasmid.

<1-6> Overexpression of thrA Gene

To increase production of O-acetylhomoserine more efficiently, overexpression of thrA gene was performed.

For this, thrA gene was PCR-amplified by using chromosome of E. coli CJM002 (KCCM10568), the L-threonine producing strain, as a template with primers of SEQ. ID. NO: 25 and NO: 26. The amplified DNA fragment was isolated by gel-elution and ligated with CJ1 promoter in pCL1920 vector using the restriction enzyme EcoRV. The ligated vector was named pCJ-thrA(M)-CL and transformed into the strains which manufactured by method of Example 1-1)˜1-5. The amino acid sequence coding by the thrA gene is represented in SEQ. ID. NO: 27 that has a mutation in amino acid position 345.

<1-7> Converting of L-threonine Producing Strain

O-acetylhomoserine producing strain was constructed using E.coli CJM002(KCCM10568) which is the L-threonine producing and L-methionine-independent strain, as described in Example 1-1 to 1-5, and named as CJM-X. The chromosome of CJM-X strain has the metX gene derived from D.radiodurans. Another L-methionine precursor producing strain was constructed using Escherichia coli FTR2533, which is the L-threonine producing strain disclosed in PCT/KR2005/00344, as described in Example 1-1 to 1-5, named as CJM-2X, and was deposited at KCCM (Korean Culture Center of Microorganism, Eulim Buld., Hongje-1-Dong, Seodaemun-Ku, Seoul, 361-221, Korea) on Dec. 9, 2003, with Accession No. KCCM10541. The chromosome of CJM-2X strain has the metX gene derived from D.radiodurans. Each strain was transformed with the thrA expression vector as described in Example 1-6 and production of methionine precursor was measured in this strain.

EXAMPLE 2 Fermentation for the Production of L-methionine Precursor

<2-1> Experiment of Flask Culture

To investigate the methionine precursor production capacity of the strain constructed in Example 1, Erlenmeyer flask culture was performed. CJM2-BTJA transformed with metX expression vector and CJM-X, CJM2-X, CJM2-X/pthrA(M)-CL were cultured on LB plate media at 31° C. for overnight. The Escherichia coli CJM-BTJA (pCJ-MetX-CL) described in PCT/KR2007/003650, O-acetylhomoserine precursor-producing strain, was deposited on Jul. 5, 2007, with the accession No. KCCM-10873.

A single colony was inoculated in 3 ml of LB medium containing spectinomycin, followed by culture at 31° C. for 5 hours. The culture solution was 200 fold diluted in 250 ml Erlenmeyer flask containing 25 ml of methionine precursor producing medium, followed by culture at 31° C., 200 rpm for 64 hours. HPLC was performed to compare with methionine precursor production capacity (Table 2 and Table 3). As a result, methionine production capacity was significantly increased in the methionine precursor-producing strain prepared from the L-threonine producing strain free from the requirement for methionine.

TABLE 1 Flask medium composition for methionine precursor production Composition Concentration (per liter) Glucose 40 g Ammonium sulfate 17 g KH₂PO₄ 1.0 g MgSO₄•7H₂O 0.5 g FeSO₄•7H₂O 5 mg MnSO₄•8H₂O 5 mg ZnSO₄ 5 mg Calcium carbonate 30 g Yeast extract 2 g Methionine 0.15 g Threonine 0.15 g

TABLE 2 Methionine precursor (O-acetylhomoserine) production by flask culture O-acetylhomoserine (g/L) CJM-BTJA/pCJ-metXdr-CL 15 CJM-X 14 CJM2-X 18.3 CJM2-X/pthrA(M)-CL 20.7

<2-2> Large Scale Fermentation

A few strains exhibiting O-acetylhomoserine production capacity was selected and cultured in a 5 L fermentor to mass-produce O-acetylhomoserine. Each strain was inoculated in LB medium containing spectinomycin, followed by culture at 31° C. for overnight. Then, a single colony was inoculated in 10 ml LB medium containing spectinomycin, which was cultured at 31° C. for 5 hours. The culture solution was 100 fold diluted in 1000 ml Erlenmeyer flask containing 200 ml of methionine precursor seed medium, followed by culture at 31° C., 200 rpm for 3-10 hours. The culture solution was inoculated in a 5 L fermentor, followed by further culture for 20-100 hours by fed-batch fermentation. The methionine precursor concentration in the fermented solution was measured by HPLC and the results are shown in Table 4.

TABLE 3 Fermentor medium composition for methionine precursor production Composition Seed media Main media Feed media Glucose (g/L) 10.1 40 600 MgSO₄7H₂0 (g/L) 0.5 4.2 Yeast extract (g/L) 10 3.2 KH₂PO₄ 3 3 8 Ammonium sulfate (g/L) 6.3 NH₄Cl (g/L) 1 NaCl (g/L) 0.5 Na₂HPO₄12H₂O (g/L) 5.07 DL-Methionine (g/L) 0.5 0.5 L-Isoleucine (g/L) 0.05 0.5 0.5 L-Threonine (g/L) 0.5 0.5

TABLE 4 Methionine precursor production in a fermentor O-acetylhomoserine (g/L) CJM-BTJA/pCJ-metXcgl-CL >55 CJM-BTJA/pCJ-metXdra-CL >75 CJM-X/pthrA(M)-CL >80 CJM2-X/pthrA(M)-CL >90

INDUSTRIAL APPLICABILITY

As described hitherto, using the methionine precursor producing strain in present invention, methionine can be produced environment-friendly than conventional chemical methionine synthetic method. And the L-methionine converted from O-acetylhomoserine produced from the strain according to the present invention can be widely used in the production of animal feed or animal feed additives, in addition to human food or food additives. 

We claim:
 1. A method for producing methionine, comprising, a) culturing a microorganism that produces and releases O-acetylhomoserine, wherein the microorganism is characterized by the following: i) a homoserine O-acetyltransferase activity (EC2.3.1.31) is introduced and enhanced by the integration of a gene from Deinococcus radiodurans that encodes homoserine O-acetyltransferase; and ii) an aspartokinase or homoserine dehydrogenase activity (EC2.7.2.4 or 1.1.1.3) is enhanced, but one or more of endogenous cystathionine gamma synthase, endogenous O-succinylhomoserine sulfhydrylase, and endogenous O-acetylhomoserine sulfhydrylase activities are reduced, inhibited or inactivated, b) producing methionine from the released O-acetylhomoserine.
 2. The method of claim 1, wherein the homoserine O-acetyltransferase has the amino acid sequence of Unipro database No. Q9RVZ8 (SEQ ID NO:32).
 3. The method of claim 1, wherein the aspartokinase or homoserine dehydrogenase peptide has a mutation at amino acid position
 345. 4. The method of claim 1, wherein the aspartokinase or homoserine dehydrogenase has the amino acid sequence of SEQ ID NO:27.
 5. The method of claim 1, wherein the microorganism is an Escherichia sp.
 6. The method of claim 1, wherein the microorganism is an Escherichia coli.
 7. The method of claim 1, wherein the microorganism is derived from L-threonine, L-isoleucine or L-lysine producing strain.
 8. The method of claim 1, wherein the activity of homoserine kinase is reduced, inhibited, or inactivated, or the microorganism does not have an endogenous gene encoding homoserine kinase.
 9. The method of claim 1, wherein a transcription regulator of methionine synthetic pathway is weakened or inactivated.
 10. The method of claim 1, wherein the microorganism is derived from the threonine producing strain Escherichia coli MF001 free from methionine-dependency having Accession No. KCCM
 10568. 11. The method of claim 1, wherein the microorganism is derived from the threonine producing strain Escherichia coli FTR2533 having Accession No. KCCM
 10541. 12. The method of claim 1, wherein the microorganism is Escherichia coli CJM-X(pthrA(M)-CL) having Accession No. KCCM 10921P.
 13. The method of claim 1, wherein the microorganism is Escherichia coli CJM2-X(pthrA(M)-CL) having Accession No. KCCM 10925P.
 14. The method of claim 1, wherein a biosynthetic pathway of threonine is suppressed.
 15. The method of claim 1, wherein expression of endogenous homoserine O-succinyltransferase encoded by metA gene, endogenous transcription regulator of methionine synthetic pathway encoded by metJ, endogenous homoserine kinase encoded by thrB gene, or a combination thereof, is suppressed.
 16. The method of claim 1, wherein the expression of thrA gene that encodes the aspartokinase or homoserine dehydrogenase is enhanced. 